Hadith: The Prophet (may Allah's peace and blessings be upon him) was asked about the unmarried bondmaid who fornicates.

عن أبي هُرَيْرة وزَيْدُ بْنُ خَالِدٍ الْجُهَنِيِّ رضي الله عنهما أنه سُئِلَ النَّبِيُّ صلى الله عليه وسلم عَنِ الأَمَةِ إذَا زَنَتْ وَلَمْ تُحْصَنْ؟ قَالَ: «إنْ زَنَتْ فَاجْلِدُوهَا، ثُمَّ إنْ زَنَتْ فَاجْلِدُوهَا، ثُمَّ إنْ زَنَتْ فَاجْلِدُوهَا، ثُمَّ بِيعُوهَا وَلَوْ بِضَفِيرٍ». قالَ ابنُ شِهابٍ: «ولا أَدري، أَبَعْدَ الثَّالِثَةِ أَوِ الرَّابِعةِ».
[صحيح] - [متفق عليه]

Abu Hurayrah and Zayd ibn Khālid al-Juhani (may Allah be pleased with both of them) reported that the Prophet (may Allah's peace and blessings be upon him) was asked about the unmarried bondmaid who fornicates, and he replied: "If she commits fornication, then flog her; if she commits fornication again, flog her again; and if she commits fornication again, flog her and sell her even for a rope." Ibn Shihāb said: "I am not sure whether the order to sell her is after the third or the fourth time."
[Authentic hadith] - [Narrated by Bukhari & Muslim]

Explanation

The Prophet (may Allah's peace and blessings be upon him) was asked about the corporal punishment prescribed by Shariah for the crime of fornication committed by the unmarried slave girl. So, he replied that she should be flogged, and she deserves to be flogged half the number of times that the free woman receives when committing fornication, which is fifty whips, as Allah, the Almighty, says: {When they are married, if they should commit adultery, they receive half the punishment of free women} [An-Nisā’: 25]. If she fornicates a second time, she should be flogged again fifty times to be deterred from committing this sin again. If she fornicates for the third time and is not deterred by that punishment and does not repent to Allah, the Almighty, and it is feared that she would do it again, then she should be flogged and sold, even at the least price, which is a cheap rope only. This is because there is no good in keeping her and there is no hope in her becoming upright, and her being far away is better than her being close so as not to be a cause of evil in the house where she resides.